Chemiluminescent method and apparatus

ABSTRACT

The invention is a chemiluminescent method and apparatus for detecting NOx (NO and NO2), SO2, CO and other gases which react with atomic oxygen to produce chemiluminescence. Atomic oxygen generated by pulse electrical discharge is added at a controlled and known flow rate to a reaction chamber to which is also added at a controlled and known flow rate a gas mixture to be analyzed. The reaction chamber is maintained at a sufficient pressure to support chemiluminescence, normally above about 0.1 Torr and in the range of about 0.5 to 5.0 Torr. Photoelectric means is used to measure the chemiluminescence, and the photoelectric means is synchronized with the pulse discharge so that photoelectric means is turned off during the time of a pulse and for sufficient time thereafter to allow the light generated by the pulse discharge to disappear, whereby the only light the photoelectric means sees is chemiluminescence. The reaction zone and the oxygen generating means are compact and are located adjacent one another to allow miniaturization of the equipment.

trite Staes att 11 1 Wooten et at.

[ CHEMILUMINESCENT METHOD AND APPARATUS [73] Assignee: Monsanto ResearchCorporation, St.

Louis, Mo.

Oct. 7, 1971 Appl. No.: 187,265

[22] Filed:

[57] ABSTRACT The invention is a chemiluminescent method and apparatusfor detecting NO, (NO and N0 S0,, C0 and other gases which react withatomic oxygen to produce chemiluminescence. Atomic oxygen generated bypulse electrical discharge is added at a controlled and known flow rateto a reaction chamber to which is also added at a controlled and knownflow rate a gas mixture to be analyzed. The reaction chamber ismaintained at a sufficient pressure to support chemiluminescence,normally above about 0.] Torr and in the range of about 0.5 to 5.0 Torr.Photoelectric means is used to measure the chemiluminescence, and thepho- [52] U.S.Cl ..250/369,23/232 E,23/254E toelectric m n i yn n z withthe pulse [51] Int. Cl. H01 j 39/00 h rge so ha photoel tric mean isturned ff during [58] Field of Search 250/83 CD, 217; he tim of a pulseand for sufficient time thereafter to 23/232 R, 232 E, 254 R, 254 E, 230PC; 73/73 allow the light generated by the pulse discharge to disappear,whereby the only light the photoelectric means [56] References Citedsees is chemiluminescence. The reaction zone and the UMTED STATESPATENTS oxygen generating means are compact and are located 3,647,3873/1972 Benson .Q 23 232 R e one another mm'amr'zauon of the 3,540,851 111970 Vrcc 23/232 E equ'pmem' 3,528,779 9/1970 Fontijn 23/254 E 4 Claims,1 Drawing Figure Primary ExaminerJames W. Lawrence Assistant Examiner-D.C. Nelms Attorney-L. Bruce Stevens, Jr

osz/zwsmpa /4 f v l 2 /5 f3 L5 /3 L e /mam "M Y wen/0' dXYGE/V l Icave/Mme k ,7 LE(/'7W5; /6

is 2:125 l2 7 5211757212) l l w r/r44 cairn 770M 5 Z Z M457. [544 l 26GF'ILTER I 4 love-5k UPf/(AL m/waw CHEMILUMINESCENT METHOD AND APPARATUSBACKGROUND OF THE-INVENTION 1. Field of the Invention The inventionrelates to the field of chemistry and specifically to an analyticalcontrol method and apparatus therefore.

2. Description of the Prior Art US. Pat. No. 3,528,779 teaches achemiluminescent method of detecting ozone wherein nitric oxide is addedat a constant flow rate to the gas mixture being tested quantitativelyfor ozone and the amount of chemiluminescent light is measured as anindication of the amount of ozone present in the gas.

US. Pat. No. 3,540,85l teaches the method for detecting trace quantitiesof any gas that, when subjected to an electrical discharge, producesoxygen atoms, e.g., sulfur oxides, carbon oxides, nitrogen oxides,molecular oxygen and other volatile oxides.

Monsanto Research Corporation under research and development ContractNo. CPA-22-69-8 with the National Air Pollution Control Administration,developed a chemiluminescence instrument for detecting NO, CO and 80:.In this instrument atomic oxygen was generated continuously by microwavedischarge in the presence of molecular oxygen. Extraneous lightincluding light generated by the atomic oxygen generator was opticallyexcluded from the reaction chamber.

SUMMARY OF THE INVENTION The invention is a chemiluminescent method andapparatus for detecting NO (NO and N S0 CO and other gases which reactwith atomic oxygen to produce chemiluminescence. Atomic oxygen generatedby pulse electrical discharge is added at a controlled and known flowrate to a reaction zone to which is also added at a controlled and knownflow rate a gas mixture to be analyzed. The reaction zone is maintainedat a sufficient pressure to support chemiluminescence, normally aboveabout 0.1 Torr and in thev range of about 0.5 to 5.0 Torr. Photoelectricmeans is used to measure the chemiluminescence, and the photoelectricmeans is synchronized with the pulse discharge so that the photoelectricmeans is turned off during the time of a pulse and for sufficient timethereafter to allow the light generated by the pulse discharge todisappear, whereby the only light the photoelectric means sees ischemiluminescence. The reaction zone and the oxygen generating means arecompact and are located adjacent one another to allow miniaturization ofthe equipment.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a schematic drawing ofthe apparatus of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An apparatus has been developedthat makes possible the construction of an on-line instrument capable ofdetecting gases that cause chemiluminescence effect when reacted withcertain atoms. The apparatus is unique in the sense that it isminiaturized and comprises an atomic generator and reaction zone as anintegrated unit, the apparatus is made of glass and includes thefollowing features:

a. Atomic oxygen is produced by an electrical discharge. To eliminateexcessive heating the discharge is actuated by a high voltage pulse ofvery low and adjustable duty cycle. The low duty cycle reduces the totalpower requirements and most important reduces heat dissipation withinthe system to a minimum. As a result, the following advantages areobtained:

1. The discharge tube can be made small and suitable for packaging.

2. Since pressure in the discharge tube must be substantially constantin order to maintain a constant operating point (stable operation) andsince AP f (AT), the operation of the discharge tube remains stable dueto the fact that no appreciable heat it being dissipated within thesystem.

3. There is a high efficiency of nascent oxygen generation.

An apparatus (described below) has been developed that integrates theatomic oxygen generator and reaction zone. Electrical discharge is usedto generate atomic oxygen. The electronic system which has beendeveloped will make the whole instrument practical and suitable for longterm stability instrumentation.

In order to make packaging practical, the generator and reactor systemare made out of small glass tubing with a small physical separationbetween the generator and the reactor. This feature produces thefollowing effects:

a. Power required for the discharge tube is high enoughto produceexcessive heating of the glass tube when continuous power is applied.

b. Electrical discharge which is luminescence by itself may enter intothe reaction chamber. The detected signal (by a photomultiplier) is muchhigher than the desired signal with the added luminescence of theelectrical discharge source. Thus elaborate means of detection arerequired to extract the wanted signal due to chemiluminescence effect.

To eliminate heating effect, a pulsating high voltage supply is used.The rate of trigger of the pulse is variable from 1 pulse per second to10 pulses per second. The duty cycle of the pulse is made to be 10percent or less (the duty cycle is also adjustable) thus reducing thepower consumption (heat dissipation) to one-tenth or less. Under theseconditions the discharge tube remains for all practical purposes atambient temperature.

To prevent the discharge signal from being detected by thephotomultiplier tube (PMT), the PMTs gating on is delayed from thedischarge pulse in time. This means that the PMT is gated on at a timelater than the falling edge of the discharge pulse. During the intervalthat the discharge pulse is on, atomic oxygen is generated. The PMT isshut off and there is no viewing of luminescence from the dischargesignal. At the time when the PMT is gated on, only the chemiluminescenceeffect is viewed since the gating on of the PMT has been delayed untilall past effect from luminescence produced by the discharge pulse hasdisappeared.

The following is a description of the apparatus of the invention andprocess based on the drawing. A sample of a gas to be analyzed isintroduced to inlet line 20 at a known and controlled rate and fromthere goes to sample inlet 12 and into the mixing and reaction zone 27.Molecular oxygen is introduced to inlet line 19 at a controlled andknown rate, passes through hollow electrode 10 and flows into atomicoxygen generator 11. Atomic oxygen is generated by pulse discharge inthe atomic oxygen generator. The other electrode for the oxygengenerator is a solid electrode 9. Pulse generator 1 provides the pulsesand is connected by wires 17 and 18 to electodes and 9, respectively.Atomic oxygen generator 11 is particularly in the form of a spiral toadd length and is made of hollow glass tubing which is sealed at eachend to an electrode. The particular atomic oxygen generator usedrequired about 1,200 volts and the pulse width used varied from about 50to 100 milliseconds. Atomic oxygen generated in atomic oxygen generator11 flows into the mixing and reaction zone 27 made of hollow glasstubing where it contacts the gas being analyzed, and chemiluminescencelight or glow is generated in intensity proportional to the quantity ofchemiluminescence reactant in the sample being analyzed, i.e. NO 80:, COand the like. From mixing and reaction zone 27 the reactants flow intooptical observation cell 7. Observation cell 7 is maintained at apressure sufficient to support chemiluminescence normally in excess ofabout 0.1 Torr and in the range of about 0.5 to 5.0 Torr by a vacuumpump not shown which pulls the vacuum on the observation cell by outletline 22 through valve 23 which communicates with observation cell 7.Vacuum gage 8 connected by line 21 to line 22 measures the pressure inthe observation cell. Optical window 26 in the observation cell allowsviewing of the chemiluminescence glow in the cell. The chemiluminescentglow is viewed through band-pass filters 6 that isolate a specificspectral band for each pollutant, e.g., when the instrument is designedto examine for NO, CO and S0,, 3 band-pass/filters are used.Photomultiplier 5 measures the intensity of the chemiluminescence glow.During the time when the pulse is being applied to the electrodes bypulse generator l photomultiplier 5 is shut off as a result ofsynchronization by delay circuit 2 which is connected to thephotomultiplier by synchronization wire and synchronization wire 13connects delay circuit 2 to pulse generator 1. In fact thephotomultiplier 5 is shut off for a time subsequent to the end of apulse sufficient to allow glow generated by a pulse in atomic oxygengenerator 11 to fade out. Then photomultiplier 5 is gated on betweenpulses to view the chemiluminescence glow in observation cell 7 throughoptical filters 6 and optical window 26. Photomultiplier power supply 4provides the power for photomultiplier 5 and is connected thereto bywire 25. The output from photomultiplier 5 can be viewed on oscilloscope3 which will show the amount of chemiluminescent glow which isproportional to the amount of NO,, CO or S0; or the like being measuredin the sample, the instrument having been calibrated with known samplescontaining known amount of NO,, CO, and S0 The oscilloscope is connectedby wire 24 to photomultiplier 5, and the oscilloscope is connected bysynchronizing wire 14 to delay circuit 2 for synchronization with thepulse generator 1. Case 16 surrounds the portions of the equipment shownin the FIGURE.

Although the invention has been described in terms of specifiedembodiments which are set forth in considerable detail, it should beunderstood that this is by way of illustration only and that theinvention is not necessarily limited thereto, since alternativeembodiments and operating techniques will become apparent to thoseskilled in the art in view of the disclosure. Accordingly, modificationsare contemplated which can be made without departing from the spirit ofthe described invention.

What is claimed is:

I. In a method for detecting in a mixture of gases the presence andconcentration of gases which react with atomic oxygen to producechemiluminescence including the steps of reacting said mixture of gaseswith atomic oxygen and measuring the chemiluminescent light generated bythe reaction of the said mixture of gases with atomic oxygen, whereinthe improvement comprises generating atomic oxygen by pulsed electricaldischarge, and gating the means for measuring the chemiluminescent lightso that said means for measuring is turned off during the time that themeans for generating atomic oxygen by pulsed electrical discharge is onand until the light from the oxygen generating pulse has disappeared soonly the chemiluminescent light is viewed.

2. A method of claim 1 wherein said electrical discharge is pulsed at arate of from 1 pulse per second to 10 pulses per second, and the dutycycle of said pulses is 10 percent or less.

3. In an apparatus for measuring chemiluminescence in gases includingmeans to produce atomic oxygen, a reaction zone including means tomaintain pressure in said zone sufficient to support chemiluminescence,means to introduce atomic oxygen into said zone, means to introduce intosaid zone a gas mixture for analysis and means to detect the quantity ofchemiluminescent light generated, the improvement comprising pulsedischarge means for producing atomic oxygen and photoelectric means todetect the quantity of chemiluminescent light generated by the gasmixture and atomic oxygen mixed in said zone, said photoelectric meansbeing synchronized with said pulse discharge means so said photoelectricmeans is turned off until the light generated by said pulse dischargemeans has disappeared and thus only chemiluminescent light is measured.

4. A miniaturized apparatus of claim 3 wherein said pulse dischargemeans produces from I to 10 pulses per second, the duty cycle of saidpulses is 10 percent or less, and said means to produce atomic oxygenand said reaction zone are located adjacent each other and are made assmall as possible.

i t ll! l 1

2. A method of claim 1 wherein said electrical discharge is pulsed at arate of from 1 pulse per second to 10 pulses per second, and the dutycycle of said pulses is 10 percent or less.
 3. In an apparatus formeasuring chemiluminescence in gases including means to produce atomicoxygen, a reaction zone including means to maintain pressure in saidzone sufficient to support chemiluminescence, means to introduce atomicoxygen into said zone, means to introduce into said zone a gas mixturefor analysis and means to detect the quantity of chemiluminescent lightgenerated, the improvement comprising pulse discharge means forproducing atomic oxygen and photoelectric means to detect the quantityof chemiluminescent light generated by the gas mixture and atomic oxygenmixed in said zone, said photoelectric means being synchronized withsaid pulse discharge means so said photoelectric means is turned offuntil the light generated by said pulse discharge means has disappearedand thus only chemiluminescent light is measured.
 4. A miniaturizedapparatus of claim 3 wherein said pulse discharge means produces from 1to 10 pulses per second, the duty cycle of said pulses is 10 percent orless, and said means to produce atomic oxygen and said reaction zone arelocated adjacent each other and are made as small as possible.